docs: def source_sum_err(self):
          """
          The uncertainty of `~photutils.SourceProperties.source_sum`,
          propagated from the input ``error`` array.
  
          ``source_sum_err`` is the quadrature sum of the total errors
          over the non-masked pixels within the source segment:
  
          .. math:: \\Delta F = \\sqrt{\\sum_{i \\in S}
                    \\sigma_{\\mathrm{tot}, i}^2}
  
          where :math:`\\Delta F` is ``source_sum_err``,
          :math:`\\sigma_{\\mathrm{tot, i}}` are the pixel-wise total
          errors, and :math:`S` are the non-masked pixels in the source
          segment.
  
          Pixel values that are masked in the input ``data``, including
          any non-finite pixel values (i.e. NaN, infs) that are
          automatically masked, are also masked in the error array.
          """
  
          if self._error is not None:
              if self._is_completely_masked:
                  return np.nan * self._error_unit  # table output needs unit
              else:
                  return np.sqrt(np.sum(self._error_values ** 2))
          else:
              return None